Abstract

The Aammeberg Zn-Pb ore district constitutes the southern margin of the Bergslagen mining district in south-central Sweden. It is hosted by a succession of Paleoproterozoic supracrustal units belonging to the Svecofennian domain and was considered as a genetically distinct ore district within Bergslagen already at the beginning of this century. The most prominent ore deposit belonging to this district is Zinkgruvan which has been a major zinc producer for more than 100 years. In this paper, the results from an investigation of the stratigraphy, geochemistry, and U-Pb and Sm-Nd isotope systematics are reported from the ca. 10-km-thick volcanosedimentary succession which forms the country, rocks in the ore district, informally named the Emme group.The Emme group may be subdivided stratigraphically into eight lithological units and generally into four parts. The lowermost part (the Moesjoen and Goekberget units and the Igelfors formation) is dominated by felsic volcanic rocks and contains the subeconomic Zn-Pb-(Cu) deposit at Marketorp. The lowermost part of the group also contains intrusions of subvolcanic rhyolitic porphyries (U/Pb age 190 + or - 18 Ma). The lowermost volcanite-dominated part grades upward into homogeneous, medium-grained arkoses (the Nrkesberg formation) of continental derivation. Two selected single zircons from these arkoses yielded 207 Pb/ 206 Pb ages of 1959 + or - 52 and 2703 + or - 178 Ma, respectively, indicating derivation from both Paleoproterozoic and Archean sources. This arkosic unit is overlain by an upper part containing felsic volcanic rocks (Mariedamm volcanic unit and its lateral correlatives, the Godegaard volcanic unit; U/Pb age 1889 (super +35) (sub -24) Ma) which locally display strong hydrothermal alteration and which are associated with deposition of the Zinkgruvan Zn-Pb-Ag ores and related carbonate rocks (the Hoeksjoen limestone). Tholeiitic subvolcanic intrusions are common in the upper part of the Emme group. The uppermost part of the Emme group is composed of a succession of sandy to muddy turbidites (the Vintergoelen formation).The epsilon Nd (1.9 Ga) values from the volcanogenic units range from 0.1 to 1.1 and are comparable with other felsic volcanic rocks elsewhere in Bergslagen. The epsilon Nd values for the arkosic units range from -2.8 to -1.9 and for the turbidites from -2.6 to +0.9. This demonstrates a rapid dilution of Archean detritus with material from younger sources in the uppermost part of the Emme group. The epsilon Nd (1.9 Ga) values for the mafic subvolcanic intrusions range from 1.4 to 4.6 and are among the highest epsilon Nd (1.9 Ga) values reported from Bergslagen. These data indicate that a primitive rift component was introduced in the upper part of the Emme group before the formation of this volcanosedimentary succession was completed. The occurrences of these mafic rift-related intrusions are considered to be genetically related to the deposition of the Zinkgruvan ore, partly by providing a separate metal source and partly by providing more heat to the convecting hydrothermal cells.The deposition of the Emme group demonstrates an interplay between voluminous, felsic volcanism and the input of large quantities of continentally derived arkosic sediments. The probable depositional site was along an active continental margin, where subsidence of the basin varied from slow and regular to very fast.All sulfide ores in the Aammeberg district were previously interpreted to constitute a characteristic ore type in Bergslagen. The present investigation demonstrates that at least two different ore-bearing levels (Zinkgruvan and Marketorp), constituting two distinct volcanosedimentary environments, can be distinguished in the Aammeberg area. It is suggested that the sulfide ores at the Zinkgruvan level represent a distinct ore-forming environment for Bergslagen, located along an active continental margin and deposited in a marine environment. The particular environment characterizing the Zinkgruvan ores appears to extend all along the southern margin of Bergslagen.

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